The present invention generally relates to automatic frequency control circuits for television signal receiving apparatuses, and more particularly to an automatic frequency control circuit (hereinafter simply referred to as an AFC circuit) for a television signal receiving apparatus, which controls a control voltage supplied to a local oscillator of a tuner depending on the frequency band of the receiving channel and on the tuning voltage of the receiving channel, so that the AFC operation is correctly carried out with respect to each of the channels.
Generally, an AFC circuit of a television signal receiving apparatus frequency-discriminates a signal which is obtained from a video intermediate frequency amplifier circuit. An output signal of the AFC circuit is passed through a lowpass filter, and is then added with a tuning voltage. The added signal is applied to a local oscillator within a tuner so as to control the local oscillation frequency of the local oscillator. As a result, an automatic frequency control (AFC) is carried out so that a video intermediate frequency assumes a predetermined frequency F.sub.VIF. The lowpass filter has an S-figure output frequency characteristic (so-called S-curve) in which the output D.C. control voltage becomes 0 volt when the video intermediate frequency assumes the predetermined frequency F.sub.VIF (58.75 MHz, for example).
The local oscillator within the tuner, receives the tuning voltage which is in accordance with the receiving channel.
The local oscillator within the tuner, receives the tuning voltage which is in accordance with the receiving channel. This tuning voltage applied to the local oscillator and the oscillation frequency of the local oscillator, are not in a linear relationship. In the bands such as the low band of the VHF, the high band of the VHF, and the UHF band, the deviation in the oscillation frequency with respect to a voltage change decreases as the tuning voltage becomes larger, and the deviation in the oscillation frequency with respect to a voltage change increases as the tuning voltage becomes smaller. In other words, the tuning voltage applied to the local oscillator and the oscillation frequency of the local oscillator, are in a non-linear relationship. On the other hand, the output control voltage of the lowpass filter changes according to the frequency of the video intermediate frequency signal. For this reason, the pull-in range of the AFC differs and the effect of the AFC accordingly differs depending on the receiving channel. For example, when receiving a channel in which the deviation in the oscillation frequency is large with respect to a change in the tuning voltage which is applied to the local oscillator, the pull-in range of the AFC is wide, and the AFC may be carried with respect to an audio carrier of that channel or with respect to a video carrier or the like of an adjacent channel. Thus, there was a possibility that the desired channel cannot be received properly.
Hence, there was a conventional AFC circuit which was designed not to carry out the AFC operation for a predetermined time when the channel was changed. However, in this conventional AFC circuit, there was a disadvantage in that it took a certain time until the AFC operation is carried out and the pull-in to the desired channel is completed after the channel was changed.